When a seller bundles together two products that have previously been sold separately, the arrangement is sometimes referred to as a tie. If the two products are bound together physically, it is often referred to as a technological tie. In the 1970s, IBM physically integrated so-called peripheral products, such as disk drives and printers which previously had been sold separately, into its central processing units. This integration was then attacked (largely unsuccessfully) by independent manufacturers of peripheral equipment as unlawful tying and as monopolization through tying. More recently, Microsoft has incorporated products, such as browsers and media players, into its operating system, behavior that again amounts to technological tying. It has also improved its server software by heightening the degree to which servers employing that software can interact. By raising the level of interaction among servers equipped with its software, Microsoft has so integrated work group servers as to enable groups of small servers to approach the capacities of mainframe computers. The European competition-law authorities see both matters as problematic. The integration of the media player has been condemned as tying; and the heightened server interaction has been faulted for failing to provide the interoperability that rival server software requires in order to participate on an equal footing with Microsoft server software in Windows work groups. Microsoft’s integration (at least in the view of the European antitrust authorities) also raises issues of essential facilities, and of the role of antitrust in achieving interoperability. Integration of these sorts - e.g., the hardware integrations of IBM and the software integrations of Microsoft - raise antitrust issues, as the extensive litigation in the United States and Europe over Microsoft’s behavior has revealed. We have now reached a moment in time in which both the American and European laws are sufficiently developed to warrant reflection and comparison. That is the task approached in this article. Part I, below, examines the European approach, exemplified in the decisions of the European Commission and the Court of First Instance about the interoperability of Microsoft’s server software. That part also examines the integration of the Windows Media Player into the Windows operating system. Part II reviews the U.S. decisions dealing with Microsoft’s integration of its Internet Explorer browser into its operating system. Since most readers will be familiar with the U.S. litigation at least in its main routlines, that review can be brief. Part III contains an initial comparison of the U.S. and European approaches to product integration. The conclusion in Part IV identifies the major differences and the doctrinal bases on which they rest.